Observations of thraustochytriaceous (Phycomycetes) and labyrinthulid (Rhizopodea) ectoplasmic nets on natural and artificial substrates—an electron microscope study
The ectoplasmic net ultrastructure of five thraustochytrids and labyrinthulids (Labyrinthula algeriensis; L. minuta; T-20, an undescribed thraustochytrid; Schizochytrium aggregation; and Thraustochytrium motivum) is described for cells grown on different microorganisms, metazoan cells, and spermatophyte cells. Net elements did not appear to actively penetrate animal cells, but a mat was formed on the surface of the food cells, lysis resulted, and net elements intermingled with the organelles, followed by food cell digestion. A similar sequence of events resulted in digestion of plant cells after penetration of the food cell wall at selected sites. The highly resistant sporopollenin layer of pine pollen was digested and penetrated by T. motivum and S. aggregatum. Internal membrane arrays of net elements in all species were found to be markedly larger than in cells grown on artificial media, thus suggesting a possible role in digestion of the food cells.The fine structure of ectoplasmic nets formed on thin formvar rafts was also examined in whole-mount preparations. The cells which use the nets in motility formed net-like arrays which were more nearly symmetrically arranged around the cells than in those species which were not motile (S. aggregatum and T. motivum). The latter species extended nets from one side of the cell and generally formed less periodically structured, smaller mesh nets around one or several amorphous, major net elements.